Drop-on-demand (DOD) liquid emission devices have been used as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators. A currently popular form of ink jet printing, thermal ink jet (or “thermal bubble jet”) devices use electrically resistive heaters to generate vapor bubbles which cause drop emission.
The printhead used in a thermal inkjet system includes a nozzle plate having an array of ink jet nozzles above ink chambers. At the bottom of an ink chamber opposite the corresponding nozzle is an electrically resistive heater.
In response to an electrical pulse of sufficient energy, the heater causes vaporization of the ink, generating a bubble that rapidly expands and ejects a drop.
There is a minimum threshold energy required to be applied to the heater in order to achieve bubble formation sufficient to reliably eject a drop. To eject a drop, the heater must supply sufficient heat to raise the ink at the heater-ink interface to a temperature above a critical bubble nucleation temperature, approximately 280 C for water-based inks. This minimum threshold energy depends on the volume of drop ejected and the printhead design such as the electrically resistive heater geometry.
Printhead designs of the prior art form the heater on an insulating thermal barrier layer, typically silicon dioxide, formed on the substrate. A protective passivation layer is formed over the electrically resistive heater for protection from the ink. When the heater is energized heat is transferred both to the ink and to the substrate. The heater in the prior art is inefficient because only about half of the energy generated by the heater goes into heating the ink. The rest flows into the substrate causing a temperature rise of the substrate. This temperature rise of the substrate is a disadvantage for high speed printing since if the substrate gets too hot, printing must be stopped to let the printhead cool down.
One mechanism for cooling the printhead is removal of heat by the ejecting drop. The amount of heat removed is proportional to the temperature and volume of the ejected drop. In fact for large drop volumes greater than 6 picoliters, printheads of the prior art can achieve a situation that for a 20-30 C temperature rise of the printhead, the energy required to eject a drop is equal to the heat energy removed by the ejected drop. In this case a steady state operating temperature can be achieved.
However, state of the art printers typically use drop sizes <3 pL. The efficiency of prior art heaters is too low for these lower volume drops to carry substantial heat energy away without the printer temperature becoming too hot. These small drops are also typically printed at a higher frequency exacerbating the problem.
Furthermore the size of the electrical drivers for the electrically resistive heaters is in part determined by the energy needed. The inefficiency of the electrically resistive heaters require larger drivers resulting in increased chip size. It is therefore desirable to increase the efficiency of the electrically resistive heater by minimizing the amount of heat that goes into the substrate.
One method to increase the efficiency of the electrically resistive heater is to provide a thermal barrier positioned between the substrate and the electrically resistive heater such as a cavity. Typically, the electrically resistive heater is formed at the end of wafer processing after the controlling circuitry has been formed. It is important therefore to design a process for forming a cavity that is compatible with low temperature backend processing.
After ejection of the ink drop it is also important that the heater cool down sufficiently so that when ink refills the chamber the temperature at the ink heater interface is insufficient to vaporize the refilling ink. Such vaporization would limit the operating frequency of the printhead. Note that while the timescale of the initial bubble vaporization is 1-2 μsec the ink refill takes place at a later time of 6-10 μsec. Therefore it is useful to provide a thermal path that can reduce the heater temperature sufficiently for this longer time cycle while at the same time not reducing the efficiency of the initial bubble formation. It is also important that this thermal path distribute the heat into the ink rather than into the substrate.
For printheads used in printing systems the energy applied to the electrically resistive heater in use is greater (typically 15-20%) than the threshold energy. This extra energy is used to account for resistance variations in the electrically resistive heaters and changes in threshold energy over the life of the heater. Because of the variations in heater resistances, this extra energy can cause variations in the drop ejection. It would therefore be useful to remove this excess heat rather than have it contribute to the vapor bubble formation.
It is also necessary for printheads to have a long lifetime. Any non-uniformities of the heater can cause poor nucleation of the vapor bubble as well as localized damage to the heater thereby reducing the lifetime of the printhead. It is therefore important that the heater surface be uniform in order to maintain the lifetime requirements of the printhead.
Damage to the heater also limits the lifetime of the printhead. Collapsing bubbles can create localized damage in the heater passivation layers. This localized damage in the passivation layers eventually reaches the heater layer, which causes a catastrophic failure of the heater. It is therefore important to limit this cavitation damage to a heater.
There is therefore a need for a printhead that has a long lifetime and provides high quality prints throughout its life. This printhead should also be capable of ejecting small drops at high frequencies with heater efficiencies adequate to prevent overheating of the printhead.